The use of refractory metals, such as molybdenum and tantalum, has been investigated for a number of applications due to the high temperature mechanical properties and high melting points of such metals. The principal disadvantage of the use of refractory metals in high temperature applications is the poor resistance of refractory metals to oxidation. Molybdenum, in particular, is subject to severe oxidation in high temperature environments.
In order to reduce oxidation, researchers have investigated the use of a variety of coating systems on refractory metal substrates. In particular, certain silicides of refractory metals, such as molybdenum disilicide, have been identified as promising coating system components for oxidation resistance. Unfortunately, in such coating systems, silicon tends to diffuse from the oxidation resistance coating into the substrate resulting in formation of less oxidation resistant sub-silicides and impaired mechanical properties, e.g., loss of ductility. Some research has addressed the possibility of synthesizing a nitrided composite film as a potential diffusion barrier layer, based on encouraging diffusion barrier characteristics reported for a nitrided molybdenum disilicide film. However, research is continuing and it is apparent that substantial challenges remain with respect to fully realizing the potential benefits of refractory metals for a variety of high temperature applications.